Hydrocarbon conversion processes often employ multiple reaction zones through which hydrocarbons pass in series flow. Each reaction zone in the series often has a unique set of design requirements. A minimum design requirement of each reaction zone in the series is the hydraulic capacity to pass the desired throughput of hydrocarbons that pass through the series. An additional design requirement of each reaction zone is the capability to perform a specified degree of hydrocarbon conversion. Designing a reaction zone for a specified degree of hydrocarbon conversion, however, often results in a reaction zone being designed larger than the minimum size required for hydraulic capacity alone. Consequently, in a hydrocarbon conversion process having multiple reaction zones with series flow of hydrocarbons, one reaction zone may have more hydraulic capacity than some other reaction zone in the series. For example, in a hydrocarbon reforming process, the last or second-to-last reforming reaction zone often has excess hydraulic capacity in comparison with the first or second reforming reaction zone.
Generally, such excess hydraulic capacity for additional throughput is not detrimental to the performance of the oversized reaction zone or any other reaction zone in the series. In theory, a process unit with extra hydraulic capacity in one or more of the reaction zones in the series could run for years with no ill effects. Nevertheless, perhaps years later when the process unit is revamped for increased throughput, an interesting debottlenecking dilemma arises: How can the extra, heretofore-unused hydraulic capacity in a large reaction zone be effectively used, in light of the fact that two or more smaller reaction zones in the series may have little or no excess hydraulic capacity?
As answers to this question, the prior art provides two debottlenecking solutions that involve re-routing the flow of hydrocarbons around two smaller consecutive reaction zones in the series. One prior art solution involves bypassing a portion (B %) of the total (100%) hydrocarbon flow through a bypass line entirely around the two smaller reaction zones, passing the remainder (100% minus B %) of the total flow of hydrocarbons in series-flow through the two smaller reaction zones in the series, combining the bypassed portion with the effluent of the second of the two smaller reaction zones, and passing the total flow of hydrocarbons through the larger reaction zone(s) only. The flow of hydrocarbons through the series can then be increased to the lesser of the combined hydraulic capacity of the smallest reaction zone and the bypass line or the smallest hydraulic capacity of the other larger reaction zone(s) of the series. The primary disadvantage of this solution, of course, is that all of the hydrocarbons that bypass one of the smaller reaction zones also bypass the other smaller reaction zone. Another disadvantage of this solution is that of the total hydrocarbons that pass through the series, only 100% minus B % passes through both of the two smaller reaction zones. Therefore, on average the hydrocarbons pass through fewer reaction zones, contact less catalyst or otherwise experience hydrocarbon conversion conditions for a shorter time, and therefore undergo less hydrocarbon conversion. Where a portion of the total flow of hydrocarbons is bypassed around more than two reaction zones, the disadvantages are compounded.
Another prior art solution involves placing two consecutive smaller reaction zones in parallel-flow rather than in series-flow, and passing only part rather than all of the hydrocarbons through each parallel reaction zone. This solution combines the smaller, parallel-flow reaction zones effectively into one large reaction zone that is in series-flow with the other larger reaction zone(s) of the series. The flow of hydrocarbons through the series can then be increased to the lesser of the combined hydraulic capacity of the parallel-flow reaction zones or the smallest hydraulic capacity of the other larger reaction zone(s) of the series. Although this second solution has an advantage in that none of the hydrocarbons that bypass one of the parallel-flow reaction zones also bypasses the other parallel-flow reaction zone, the disadvantage of this second solution is that none of the total hydrocarbon flow through the series passes through both of the two smaller reaction zones. The more smaller reaction zones placed in parallel, the greater are the disadvantages of this second solution.
Consequently, a method is sought for passing hydrocarbons through multiple reaction zones where a portion of the total reactant flow must be bypassed around two or more consecutive reaction zones, but where nevertheless the detrimental effects on hydrocarbon conversion are minimized. The method must prevent hydrocarbons that bypass one of the reaction zones from also bypassing the next reaction zone in the series. Furthermore, the method must maximize the total amount of hydrocarbon that passes through all of the reaction zones that are bypassed.